Fail safe thermostatic switch

ABSTRACT

Disclosed is a temperature responsive switch including a pair of contacts that open and close in response to temperature variations and are connected to lead wires by fusible coupling such as solder. In the event of contact overheating one or both of the fusible couplings melts and a bias spring separates the affected lead wire and contact, thus opening the switch.

United States Patent [191 Poitras Mar. 12, 1974 [54] FAIL SAFE THERMOSTATIC swrrcn 3,304,396 2/1967 Hasson 331/407 x 1 1 [75] Inventor: Edward J. Poitras, Holhston, Mass. $32 2 I I 73 Assignee; Walter Kidde & Company, Inc 2,307,867 1/1943 Smith 337/399 C11fton, N.Y. FOREIGN PATENTS OR APPLICATIONS [22] Filed: Sept. 5, 1972 201,788 8/1923 Great Britain 337/239 678,369 1 1964 C d 337 239 21 App]. No.: 286,154 7 I a Primary Examiner-Velodymyr Y. Mayewsky [52] U.S. Cl 337/239, 337/399, 337/407, Attorney, Agent, or Firm-John E. Toupal 337/412 [51] Int. Cl. H011! 85/36 57 ABSTRACT [58] Field of Search 337/239, 382, 388, 389, l 1 337/392, 399 406, 407, 410 412 some 15 a temperature responslve svntc me u mg a pan of contacts that open and close in response to [561 'tiaz az zzzbizifizzz521:2;551225 5322121:

UNITED STATES PATENTS of contact overheating one or both of the fusible cou- 3,639,874 2/1972 Deelman 337/239 X plings melts and a bias s ring separates the affected 2,257,990 10/1941 Turenne 337/392 lead wire and Contact thus opening the switch 2,619,567 11/1952 Walbndge... 337/389 3,673,536 6/1972 Kondo 337/392 12 Claims, 4 Drawing Figures PATENTEBHAR 12 m4 SHEEI 1 [IF 2 PATENTEDIAR 12 I974 SHEET 2 (IF 2 FAIL SAFE THERMOSTATIC SWITCH BACKGROUND OF THE INVENTION This invention relates to temperature responsive switches and, more particularly, to such switches that incorporate auxiliary switching apparatus that is activated in the event of switch overload or failure of the primary switching apparatus.

Temperature responsive switches are widely used in electrical temperature control devices of various types. Often control is achieved by utilizing the deformation resulting in a combination of dissimilar metals as temperature changes. For example, a well-known type of switch is the tension switch in which the ends of an arcuate element are secured to the ends of a linear element. One of the elements is formed of metal with a high coefficient of expansion and the other element is formed of metal with a low coefficient of expansion. Therefore, as the temperature changes the radius of curvature of the arcuate element changes and the central portion thereof moves in a path substantially perpendicular to the linear element. A contact mounted on the arcuate member engages and disengages another contact during the reciprocating perpendicular motion and thus provides temperature responsive switching. Prior switches of this type are disclosed in US. Pat. Nos. 2,307,867, 2,257,990, and 2,090,407.

A difficulty encountered in the use of a tension switch is that contact movement is not rapid or great. Short contact travel, coupled with high current requirements can in isolated instances cause arcing and subsequent welding of the contacts in a closed position. In such cases overheating of the controlled apparatus can occur since the welded contacts continue to supply power thereto. Similar problems can result from the inadvertent use of thermostatic switches with operating temperature settings higher than control temperatures desired.

The object of this invention, therefore, is to provide a temperature responsive switch with overload protection apparatus that positively opens the switch in the event of overheating caused by failure of the primary contacts to operate properly.

SUMMARY OF THE INVENTION This invention is characterized by a temperature responsive switch including a pair of electrical contacts that open and close in response to temperature transitions spanning a contact temperature. Lead wires running from the contacts connect the switch to the controlled circuit. The lead wires are attached to the contacts by a fusible coupling that melts at a preselected fuse temperature that is higher than the contact temperature. A bias mechanism urges each lead wire away from the contacts so as to disconnect it therefrom when the fusible coupling melts. In the event of overheating caused by failure of the contacts to open at a desired contact temperature, the fusible coupling melts allowing the bias mechanism to separate the lead wire from the contact and thereby open the circuit.

A feature of the invention is the utilization of a fusible coupling such as solder to connect the lead wires to the contacts and the inclusion of a spring as a bias apparatus. Such additions to the switch add little to the cost and complexity, but provide the overload or overheat protection described above. In addition, this con- 2 figuration enables the switch to be repaired after arcing by resoldering the lead wire to the contact.

Another feature of the invention is its incorporation into a tension switch wherein the coupling between the arcuate members and the linear member is adjustable thereby permitting adjustment of the preselected contact temperature. Provision for temperature adjustment renders the switch useful for a variety of applications. In any case, however, the fusible coupling selected should have a melting point higher than the preselected contact temperature of the switch.

DESCRIPTION OF THE DRAWINGS These and other features and objects of the present invention will become more apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein;

FIG. 1 is a sectional view of the subject switch assemy;

FIG. 2 is a sectional view of the switch shown in FIG. 1 taken along the line 22;

FIG. 3 is a sectional view taken along the line 3-3 showing the switch contacts and fusible couplings in greater detail; and

FIG. 4 is a fragmentary, broken away view of the interior of the switch after the fusible coupling has melted.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG; 1 there is shown a temperature responsive switch 21 that is connected to an external circuit by a first lead wire 22 and a second lead wire 23. A linear cylindrical casing element 24, a first end cap 25 and a second end cap 26 form a unitary capsule. The casing 24 is fabricated from ametal with a relatively high coefficient of expansion as, for example, brass. A first switch contact 27 is supported by a first arcuate member 28 and a second metal contact 29 is supported by a second arcuate member 31. Each of the arcuate members 28 and 31 is formed of a metal with a low co efficient of expansion as, for example, a nickel-iron alloy so as to function as a temperature responsive actuator for the contacts 27 and 29. One end of each of the arcuate members 28 and 31 is anchored in an attachment block 32 that is mounted on a shaft 33 and restrained by a flange 34 thereon. The shaft 33 is locked in the end block 25 by a pin 35. The opposite end of each of the arcuate members 28 and 31 is clamped in a second attachment block 36, the interior of which is threaded. An adjustment screw 37 is received by the threaded interior. The smooth shank 38 of the adjustment screw 37 rotates freely in a smooth hole in the end cap 26 and the inward motion of the screw is limited by a head 39. The spring action of the arcuate members 28 and 31 draws the attachment blocks 32 and 36 together and thus the head 39 of the attachment screw 37 is held against the end cap 26. A screw driver can be inserted in a slot 41 in the attachment screw 37 and rotation thereof adjusts the spacing between the attachment blocks 32 and 36.

Referring now to FIG. 2 it is seen that two ribs 45 on the interior of the casing 24 are received by mating slots 46 in the attachment block 36 and thus the block cannot rotate as the attachment screw is turned. As shown in FIG. 3, the contact 27 is supported by a contact mount 51 that is wrapped around the arcuate 3, member 28 but is separated therefrom by an insulator 52. Similarly, the contact 29 is mounted by a contact mount 53 that is separated from the arcuate member 31 by an insulator. Y

The conductive portion 55 of the lead wire 22 is connected to the contact mountSl by a fusible coupling 56 such as, for example, solder. The coupling 56 melts at a temperature selected to be higher than the highest anticipated operating temperature of the switch 21. Similarly, the conducting portion 57 of the lead wire 23 is connected to the contact mount 53 by a fusible coupling 58. Each of the wires 22 and 23 is wrapped around the screw 37 at least one turn to minimize stress exerted on the contact mounts 51 and 53. Anchored in theattachment block 36, and urging the wire 22 away from the contact 27 is a bias spring 61. Similarly mounting and urging the lead wire 23 from the contact 29 is a second bias spring 62. A cylindrical insulator 63 covers the interior of the casing 24 in the vicinity of the contacts 27 and 29.

During construction of the switch 21 the various parts are assembled in the absence of the cylindrical casing 24 and the end cap 25. The adjusting screw 37 is set so that the attachment block 36 is toward the left end 64 thereof. Next, the cylindrical casing 24 is slipped into place and the end cap 25 is placed with the shaft 33 passing therethrough. The switch 21 is locked together by insertion of the pin 35. Inasmuch as the attachment block 36 is near the left end 64 of the adjustment screw 37, the attachment block 32 is located near the center of the shaft 33 rather than against the flange 34. Thus, positioning of the shaft 33 to receive the pin 35 is simplified.

Equilibration of the switch 21 at the preselected contact temperature begins calibration. When equilibration is complete the cylindrical casing 24 is at the preselected temperature and thus, at its corresponding preselected critical length. The adjustment screw 37 is turned in a clockwise direction to draw the attachment block 36 toward the end .cap 26 and the block 32 toward the flange 34. When the attachment block 32 reaches the flange 34 and thus can travel no further the contacts 27 and 29 are separated as the attachment blocks 32 and 36 are separated straightening the arcuate members 28 and 31. Thus, to complete calibration at the preselected contact temperature the adjustment screw 37 is rotated until the contacts 27 and 29 just separate as registered by an ohmmeter connected to the lead wires 22 and 23.

During use of the switch 21, ambient temperatures above the preselected contact temperature cause the cylindrical casing 24 to expand beyond its critical length and straighten the arcuate members 28 and 31 thus separating the contacts 27 and 29. Conversely, a subsequent drop in ambient temperature to below the preselected contact temperature reduces the length of the cylindrical casing 24. This moves the contacts 27 and 29 together and further reductions in the length of the cylindrical casing 24 cause the attachment block 32 to move on the shaft 33 toward the end block 25. Thus, the switch cycles between open and closed positions in response to changes in ambient temperature spanning the preselected contact temperature.

Assuming, however, that overheating of the contacts 27 and 29 occurs, the solder couplings 56 and 58 melt to release the leads 22 and 23. Overheating could be caused, for example, by failure of the contacts 27 and 29 to open at a desired control temperature thereby providing a continuous flow of power to a heating unit and a resultant excessive rise in ambient temperature. The contact failure could result from an inadvertently high contact temperature setting or from a mechanical failure.

Upon melting of the fusible coupling 56, the bias spring 61 forces the lead wire 22 against the insulation 63 and thereby positively disconnects the leads 22 and 23. Similarly, melting of fusible coupling 58 allows the bias spring 62 to force the wire 23 against the insulation 63. Thus, the circuit being controlled is automatically opened in response to any inadvertent overheating of the switch 21..

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, although the invention is particularly useful with a tension type switch it will be obvious that the overheat protection concept disclosed can be desirably incorporated into other types of thermostatic switches. It is therefore, to be understood that within the scope of the appended claims the invention can be practised otherwise than as specifically described.

What is claimed is:

1. A thermally responsive switch comprising:

electrical contact means for opening and closing an electrical circuit;

temperature responsive actuator means for opening and closing said contact means in response to temperature transitions spanning a preselected contact temperature; lead means for connecting said contact means to an electrical circuit; fusible coupling means connecting said lead means to said electrical contact means and exposed to the same ambient temperature therewith, said fusible coupling means adapted to melt at a preselected fuse temperature that is higher than said contact temperature; and separation means for insuring permanent separation of said lead means and said electrical contact means for producing disconnection therebetween when said fusible coupling means fuses.

2.. A switch according to claim 1 wherein said separation means comprises bias means disposed so as to urge separation of said lead means and said electrical contact means for producing disconnection therebetween when said fusible coupling means fuses.

3. A switch according to claim 2 wherein said fusible coupling means is in thermal contact with said contact means and is responsive to the temperature thereof.

4. A switch according to claim 3 wherein said electrical contact means comprises a metal contact and said lead means comprises a lead wire and said fusible coupling means comprises a fusible metal for affixing said lead wire to said metal contact.

5. A switch according to claim 4 wherein said bias means comprises a spring for separating said lead wire from said metal contact when said fusible metal melts.

6. A switch according to claim 5 wherein said actuator means comprises a metal linear actuator with a given coefficient of thermal expansion and further comprises an arcuate metal member with a lower coefficient of thermal expansion that supports said metal contact, the ends of said arcuate member being secured to the ends of said linear actuator by attachment means and said metal contact being mounted on said arcuate member so as to move in a path substantially perpendicular to said linear actuator as the length thereof changes in response to temperature.

7. A switch according to claim 6 wherein said contact means comprises a second metal contact disposed to said substantially perpendicular path of said metal contact so as to be connected and disconnected therewith in response to changes in length of said actuator.

8. A switch according to claim 7 wherein said attachment means comprises adjustment means so that said preselected critical length and said preselected contact temperature are adjustable.

9. A switch according to claim 7 comprising casing 6 means for enclosing the components thereof.

10. A switch according to claim 9 wherein said casing means comprises'said linear actuator means.

11. A switch according to claim 7 wherein said lead means comprises a second lead wire and said fusible coupling means comprises a second fusible metal for affixing said second lead wire to said second metal contact and said bias means comprises a second spring for separating said second lead wire and said second metal contact when said second fusible metal melts.

12. A switch according to claim 11 comprising a second opposing arcuate member that is secured by said attachment means and wherein said second metal contact is mounted on said second arcuate member and travels in said substantially perpendicular path. 

1. A thermally responsive switch comprising: electrical contact means for opening and closing an electrical circuit; temperature responsive actuator means for opening and closing said contact means in response to temperature transitions spanning a preselected contact temperature; lead means for connecting said contact means to an electrical circuit; fusible coupling means connecting said lead means to said electrical contact means and exposed to the same ambient temperature therewith, said fusible coupling means adapted to melt at a preselected fuse temperature that is higher than said contact temperature; and separation means for insuring permanent separation of said lead means and said electrical contact means for producing disconnection therebetween when said fusible coupling means fuses.
 2. A switch according to claim 1 wherein said separation means comprises bias means disposed so as to urge separation of said lead means and said electrical contact means for producing disconnection therebetween when said fusible coupling means fuses.
 3. A switch according to claim 2 wherein said fusible coupling means is in thermal contact with said contact means and is responsive to the temperature thereof.
 4. A switch according to claim 3 wherein said electrical contact means comprises a metal contact and said lead means comprises a lead wire and said fusible coupling means comprises a fusible metal for affixing said lead wire to said metal contact.
 5. A switch according to claim 4 wherein said bias means comprises a spring for separating said lead wire from said metal contact when said fusible metal melts.
 6. A switch according to claim 5 wherein said actuator means comprises a metal linear actuator with a given coefficient of thermal expansion and further comprises an arcuate metal member with a lower coefficient of thermal expansion that supports said metal contact, the ends of said arcuate member being secured to the ends of said linear actuator by attachment means and said metal contact being mounted on said arcuate member so as to move in a path substantially perpendicular to said linear actuator as the length thereof changes in response to temperature.
 7. A switch according to claim 6 wherein said contact means comprises a second metal contact disposed to said substantially perpendicular path of said metal contact so as to be connected and disconnected therewith in response to changes in length of said actuator.
 8. A switch according to claim 7 wherein said attachment means comprises adjustment means so that said preselected critical length and said preselected contact temperature are adjustable.
 9. A switch according to claim 7 comprising casing means for enclosing the components thereof.
 10. A switch according to claim 9 wherein said casing means comprises said linear actuator means.
 11. A switch according to claim 7 wherein said lead means comprises a second lead wire and said fusible coupling means comprises a second fusible metal for affixing said second lead wire to said second metal contact and said bias means comprises a second spring for separating said second lead wire and said second metal contact when said second fusible metal melts.
 12. A switch according to claim 11 comprising a second opposing arcuate member that is secured by said attachment means and wherein said second metal contact is mounted on said second arcuate member and travels in said substantially perpendicular path. 